1. Field of the Invention
The present invention relates to a method for bleaching cellulose fiber material with hydrogen peroxide. More particularly, the present invention relates to the aforesaid method in which a stabilizer for hydrogen peroxide in an aqueous bleaching solution for bleaching fiber materials.
2. Description of the Related Art
It is known that a bleaching process using hydrogen peroxide as a bleaching agent is carried out under an alkaline condition. However, it is also known that, under the alkaline condition, if the bleaching solution contains a heavy metal, for example, iron, copper or manganese, hydrogen peroxide in the bleaching solution is decomposed to a certain extent, due to the undesirable influence of the heavy metal, in accordance with the following chemical reaction: EQU 2H.sub.2 O.sub.2 .fwdarw.2H.sub.2 O+O.sub.2
the above-mentioned phenomenon hinders the production of perhydroxyl ions (OOH.sup.-), which make a large contribution to the bleaching effect of hydrogen peroxide, in accordance with the following reaction: EQU H.sub.2 O.sub.2 .fwdarw.H.sup.+ +OOH.sup.-
and results in an undesirable decrease in the bleaching effect of hydrogen peroxide.
Usually, the above-mentioned undesirable decomposition of hydrogen peroxide into molecular oxygen is prevented by adding a stabilizer to the hydrogen peroxide-containing bleaching solution. A well-known conventional stabilizer for the hydrogen peroxide bleach is sodium silicate, which exhibits an excellent stabilizing effect and allows fiber materials having an excellent whiteness to be produced. However, sodium silicate is disadvantageous in that it causes water-insoluble silicate compounds to be produced in the bleaching solution, which are deposited in the form of scales not only on the surface of the bleached fiber material but also on a bleaching apparatus, and thus degrade the touch of the bleached fiber material and the function of the bleaching apparatus. As other stabilizers for the hydrogen peroxide bleach, organic chelating agents, for example, ethylenediamine tetraacetic acid (EDTA) and diethylenetriamine pentaacetic acid (DTPA), and inorganic chelating agents, for example, polyphosphates, are disclosed in "MOL", No. 4, page 42, 1968, and water-soluble protein substances, for example, soybean protein and casein, are disclosed in Japanese Examined Patent Publication No. 50-34675 (1975).
The above-mentioned stabilizers different from sodium silicate exhibit a certain stabilizing intensity and an industrial usefulness when used in a batch type bleaching systems at a large liquor ratio. However, these stabilizers are disadvantageous in that, when used in a continuous bleaching system at a relatively small liquor ratio, the resultant hydrogen peroxide-stabilizing effect, whiteness of the bleached fiber material, prevention of degradation of the bleached fiber material, and process stability are poorer than those of sodium silicate. Accordingly, the above-mentioned conventional stabilizers other than sodium silicate do not satisfy the requirements of the bleach industry.
Japanese Unexamined Patent Publication No. 52-103,386 (1977) discloses that a bleaching stabilizer consisting of a poly(sodium-.alpha.-hydroxyacrylate) exhibits an excellent stabilizing effect, whiteness-increasing effect, and other effects, at a very high level, which could not be obtained by the conventional stabilizer. That is the poly(sodium-.alpha.-hydroxyacrylate) exhibits an excellent stabilizing effect for the hydrogen peroxide bleach and inhibition of the decomposition of hydrogen peroxide due to the presence of a hydrogen peroxide-decomposing catalyst, for example, heavy metals, at a high level comparable to those of sodium silicate. Also, it has been found that the poly(sodium-.alpha.-hydroxyacrylate) provides an excellent protection of the fiber materials from the degradative action of the bleaching agent. However, it has been further found that the stabilizing effect of the poly(sodium-.alpha.-hydroxyacrylate) is excessively high, and thus sometimes restricts the bleaching effect and cotton seed-removing effect of the hydrogen peroxide.
Furthermore, the poly(sodium-.alpha.-hydroxyacrylate) is very expensive and, therefore, if a stabilizer consisting of this compound alone is used in a necessary amount for exhibiting a desired bleaching effect, the bleaching process becomes extremely costly, and thus cannot be industrially utilized. In order to avoid the above-mentioned disadvantages, Japanese Unexamined Patent Publication No. 55-76161 (1980) discloses an improved hydrogen peroxide bleaching process in which a composite stabilizer consisting of a poly(sodium-.alpha.-hydroxyacrylate) combined with a polyphosphate is used. However, this type of composite stabilizer is disadvantageous in that the resultant bleached fiber material exhibits an unsatisfactory touch and is not preferable for industrial use.
Also, attempts have been made to use a composite stabilizer consisting of a poly(sodium-.alpha.-hydroxyacrylate) combined with sodium silicate. However, the results obtained by usage of the composite stabilizer were not satisfactory.